Project Abstract Adolescents can benefit greatly from evidence-based, medically-accurate sexual and relationship health education. However, both teachers and parents face challenges in effectively teaching about sexual health and relationships leading to students receiving incomplete or inaccurate information. Young people turn to the media for information, and many media messages about sex and relationships are inaccurate and unhealthy. Consumption of unhealthy media messages (e.g., those that normalize or glamorize risky sex) is related to early and risky adolescent sexual behaviors. Media literacy education (MLE), which teaches critical analysis of media messages, has been shown to be an effective intervention for changing risky health behaviors including those related to sexual and relationship health. Thus, this Phase II application proposes to complete the development of and test the efficacy of Media Aware (MASH-HS), an interactive web-based sexual health MLE program, designed to develop high school students' critical thinking skills and promote sexual and relationship health. It is hypothesized that this program will enhance adolescents' media literacy skills, sexual health knowledge, cognitions (i.e., beliefs, self-efficacy, and norms), and behaviors regarding healthy decision-making. The program is highly innovative due to inclusion of a broad range of sexual and relationship health topics; attention to media influence, a risk factor often ignored in sexual risk prevention programs; and a paradigm shift for school-based sexual health education away from teacher-led programming towards a self-paced, online format that enhances student learning. The Phase I Feasibility Study revealed significant impacts of the program on students, including enhanced critical thinking about media messages; increased ability and intentions to intervene if they were a bystander to a possible sexual assault; decreased normative beliefs about the frequency of teen sexual activity and risky sexual activity; and decreased willingness to engage in risky sexual behaviors. The first aim of Phase II is to make edits to the program and Learning Management System to enhance program implementation with help from an Advisory Panel of teachers and school administrators. The second aim of Phase II is to conduct a randomized control trial to examine the program's impact on factors related adolescents' sexual and relationships health where schools (n=16) are randomly assigned to condition, and all of one teacher's classrooms from each school will either receive MASH-HS (n=40 classes) or their standard health education curriculum (n=40 classes) between pretest and posttest questionnaires. The RCT will also include a 3-month follow-up in order to examine if the program results in behavioral changes. The potential commercial application is an engaging, cost-effective sexual and relationship health program. One added benefit is that while enhancing health, the program develops critical thinking skills, highly valued in education settings, which should contribute positively to program adoption and scalability.